Corrosion products stemming from the primary heating system, which to a major extent comprises the tubes and pipe-lines of the steam generators, are conveyed into the reactor core where they are deposited on the fuel elements.
After some time, the corrosion products, which are now radioactive after the neutron irradiation, are liberated from the fuel elements and are subsequently deposited on the parts of the primary system in contact with water which lie outside the reactor core. Then the radioactive corrosion products give rise to radiation fields outside the core and thereby to radiation doses to the operational personnel.
Another cause of the occurrence of radiation fields is fuel element leakage. In case of leakage in the encapsulating material of the fuel elements fission products are leached out by the circulating water. These products are then incorporated in the oxide layers on the parts of the system (primarily the steam generators) lying outside the reactor core.
The radiation doses received by personnel must be kept within prescribed limits. For reasons of health and operational economy, the doses should of course be kept as low as is reasonably possible.
Before undertaking major work on the primary system, it can thus be desirable to remove the radioactive corrosion and fission products which have been deposited on the primary system surfaces. By a partial or complete dissolution of the oxide layers, a substantial portion of the radioactive isotopes can be removed from the system surfaces. In nuclear reactor terminology this process is denoted decontamination. Most of the known processes within this technology have been described in detail in J. A. Ayres, Ed., Decontamination of Nuclear Reactors and Equipment, the Ronal Press Company, N.Y. (1970).
During the years from about 1961 and up to the first years of the seventies, only a very small number of decontaminations of reactor systems were carried out. The most discussed decontaminations during this period were those of the Shippingport PWR (PWR=Pressurized Water Reactor) in the USA and the PWR plant at Greifswald in the GDR. Modified versions of the APAC method developed during 1961 in the USA were used in these decontaminations.
There were two steps in this method, namely a first oxidizing step with alkaline permanganate followed by a second dissolving step with an acidic decontamination solution containing ammonium citrate.
Common to all modifications of the APAC process is that the contents of chemicals must be relatively high for acceptable decontamination factors to be achieved. The decontamination factor (Df) is defined in the following way: ##EQU1##
In occasional cases where the APAC process has been used, it has been necessary to repeat the decontamination a number of times to obtain a satisfactory result.
The radioactive solutions of chemicals from this process have either been purified by ion exchangers or been treated in special evaporators. The greatest disadvantage with the APAC process is the large volumes of waste occurring in the form of radioactive ion exchange masses or evaporator residues.
The above-mentioned disadvantages have resulted in that during 1970 work was started in several quarters on developing new processes. The aim then was to achieve processes which:
provide an acceptable decrease of the radiation fields during a treatment time of maximum 36 hours,
only require low concentrations of chemicals in the final step by the utilization of continuous regeneration of the chemicals with cation exchange,
are possible to perform at temperatures below 100.degree. C., give a final waste in the form of ion exchange masses containing all chemicals present, including metals and radio isotopes released or liberated during the process.
As to the processes which began to be developed during the seventies and which are used today, it has been found necessary to include a pre-treatment step with oxidizing reactants.
In said pre-treatment step essentially the following oxidizing agents are used:
permanganate in an alkaline or nitric acidic environment (in the latter case the pH is about 2.5)
potassium hexacyanoferrate in an alkaline environment.
In the subsequent treatment step there are used almost exclusively organic acids (citric or oxalic acids and ammonium salts of these) and some strong complex forming agent, e.g. EDTA (ethylenediaminetetraacetic acid). Additives in the form of reducing agents such as aldehydes or ascorbic acid can also be present in the acid treatment step.
The conditions (reducing, high pH) prevailing in a pressurized water reactor are such that the oxide layers formed will to a large extent have relatively high contents of chromium, partially together with nickel, in the form of oxide or spinel phases. To have these oxide layers dissolved at all in organic acids, it is thus necessary to carry out the pre-treatment in an oxidizing environment. At present the completely dominating oxidation agent in this respect is permanganate. The reaction sequence for the oxidation step is substantially as follows: EQU 3MnO.sup.-.sub.4 +Cr.sup.3+ +8H.sub.2 O.sup..fwdarw. .rarw.3Mn.sup.2+ +5CrO.sub.4.sup.2- +16H.sup.+
In order to illustrate more in detail the decontamination effect, which may be obtained by the processes available today, reference is made to the following.
In all processes available today in Europe, USA and Canada there are at least two treatment steps, one of which is always the above-mentioned pre-oxidation step. All these processes have been tested, partly on a laboratory scale, partly at half or full scale in some cases. The processes worked out in Europe have been tested in two international decontamination projects. These are the Agesta decontamination project in process in Sweden, and the project in process at the Pacific National Laboratories, Richland, Wash., USA. The tests in the USA have been carried out in an authentic steam generator taken from the Surry-II PWR plant after an approximate operation time of 6 years. In the Agesta project, laboratory tests have been carried out on samples taken from the steam generators in Ringhals-2 (Sweden), Biblis A (Germany), Millstone 2 (USA) and from the inlet chamber in one of the steam generators in the Borssele reactor in Holland.
The Swedish laboratory tests have been carried out with so-called "soft" processes (i.e. processes where low contents of chemicals are used) developed at:
Studsvik Energiteknik AB (Sweden) PA1 Kraftwerk Union (Germany) PA1 EIR (Switzerland) PA1 BNL (CEGB) (England)
The samples from the above-mentioned PWR:s were of the following materials:
______________________________________ Ringhals-2 Inconel 600 Millstone-2 Inconel 600 Bib11s A IncoIoy 800 Borssele AISI 304. ______________________________________
In this connection it may be mentioned that the compositions of these materials in percent per weight are:
______________________________________ Material C Si Mn Cr Ni Mo Fe ______________________________________ AISI 304 0.04 0.4 1.2 19 9.5 0.2 residue Incoloy 800 0.02 0.6 0.6 21 33 residue Inconel 600 0.02 0.3 0.8 16 73 residue ______________________________________
The results of these tests can be summarized as follows:
The samples of Inconel 600 were difficult to decontaminate. Decontamination factors exceeding 3 (the lowest acceptable value) could only just be achieved by three of the four processes.
The samples of Incoloy 800 and AISI 304 reached satisfactory decontamination factors by a good margin.
In the tests in the steam generator from Surry-II PWR, a process was tested which had been developed in Canada as well as a process similar to the one tested by BNL (CEGB) in the Agesta project.
The results of the tests showed here as well that surfaces of Inconel 600 were very difficult to decontaminate. Acceptable decontamination factors could be achieved only after several treatment cycles. It should be noted in this connection that a pre-oxidation step with permanganate is included in both these processes.
As prior art in this area, even if this art is not utilized in practice today, the art disclosed in the Swedish Patent Application Ser. No. 8001827-8 (based on U.S. Ser. No. 028,200 filed on Apr. 9, 1979) now U.S. Pat. No. 4,287,002, may also be mentioned. Said patent application describes a decontamination method where the pre-oxidation step is carried out by means of ozone as the oxidation agent. In the subsequent acid dissolving step organic acids and complex forming agents are used at high temperatures such as 85.degree. C. and 125.degree. C. In the patent application there are described decontamination trials on samples pre-oxidized for 7 days (PWR environment at 350.degree. C.) and thereafter exposed for 3 months at 250.degree. C. in a PWR trial plant. In the trials, decontamination factors with a mean of about 2.7 were obtained for samples of Inconel 600, which must be regarded as a low value.